Certain partial oxidation products are produced commercially by the oxidation of ethylene or propylene in the vapor phase over a suitable catalyst. For example, ethylene oxide and vinyl chloride are produced by the partial oxidation of ethylene with oxygen in the presence of selected catalysts, and acrylonitrile, propylene oxide and acrolein are produced by the partial oxidation of propylene with oxygen in the presence of selected catalysts. Air is generally used as the source of oxygen, because of its low cost and ready availability. The reaction can be carried out in any suitable reactor and it produces the desired partial oxidation product and generally carbon monoxide, carbon dioxide and water as byproducts. The reaction conversion is usually less than 100%; accordingly the reactor effluent also generally contains unreacted ethylene or propylene. Furthermore, industrial grade ethylene and propylene usually contains small amounts, for example up to about 10% by volume, ethane and propane, respectively. Therefore, since ethane and propane is not generally affected by ethylene and propylene partial oxidation catalysts, the effluent usually contains ethane or propane. In addition to these components the effluent will contain nitrogen and argon if air is used as oxidant.
In the past it was common to practice the above-described partial oxidation processes on a single-pass basis, with the conversion of hydrocarbon to the desired product being maximized. This resulted in low overall efficiency, since the selectivity to the desired partial oxidation product is below the maximum at high conversion. Consequently, the concentration of carbon oxides in the reactor effluent was high. The effluent stream was usually incinerated, so that the only return realized from the effluent was heat value from combustion of the hydrocarbon and carbon monoxide.
Later improvements included recycling part of the gaseous effluent to the reactor after recovery of the desired partial oxidation reaction product(s). This made it possible to lower hydrocarbon conversion and increase selectivity to the desired product. These improvements resulted in a reduced "per pass" conversion but the overall efficiency of the process was increased. However, in such recycle processes, part of the effluent remaining after recovery of the desired partial oxidation product(s) had to be purged from the system to prevent the build-up of carbon oxides and nitrogen and alkanes, such as ethane and propane.
Because of the difficulty of separating ethylene from ethane and propylene from propane efficient operation of recycle ethylene and propylene partial oxidation processes is hard to achieve when the alkene feed contains the corresponding alkane as an impurity. Poor separation of ethylene from ethane and propylene from propane in the post product recovery zone can result in a buildup of ethane or propane in the overall process. Continuous efforts are underway to enhance the efficiency of recycle partial oxidation processes. These efforts include investigations for improved procedures for separating the alkene from the corresponding alkane prior to recycling the alkene to the reactor. The present invention provides such an improved procedure.